SURFACE VIBRATIONS OF ARGON CLUSTERS BY HELIUM ATOM SCATTERING

In a crossed molecular beam arrangement helium atoms are scattered fro m argon clusters which are generated in the size range of <(n)over bar = 23> to <(n)over bar = 4600> by adiabatic expansion with conical noz zles. The inelastic energy transfer is detected by time-of-flight anal ysis of the scattered helium atoms with a resolution of better than 1 meV. The angular dependence allows to distinguish between single and m ultiphonon excitations. The measured peaks of the averaged transferred energy in the single phonon regime decrease with increasing average c luster size and converges to the values of the surface phonons of soli d argon. This behavior is explained by treating the data as collective breathing vibrations of a solid sphere. (C) 1997 American Institute o f Physics.